Metallic building structure



H. R. RASCH Dec. 22, 1964 1/ NT HERMAN R. RASCH Ska- '1 W A TTORNE Y6:

FIG 2 Dec; 22, 1964 scl-l 3,162,278

METALLIC BUILDING STRUCTURE Filed May 8, 1961 2 Sheets-Sheet 2 I L- V/ l is ) m INVENTOR.

HERMAN R. RASCH A TTORNE Y5:

United States Patent Office 3,162,278 Fatented Dec. 22, 1964 3,162,278 METALLIC BUILDING STRUtZTURE Herman R. Rasch, Terre Haute, 1nd,, assignor to National Steel Corporation, a corporation of Delaware Filed May 8, 1961, Ser. No. 188,472 4 Claims. ((Il. 189-1) The present invention relates to metallic building structure, and more particularly to such structure comprising rigid portal frames of the type often used in relatively wide, low-pitch rigid frame buildings.

Rigid frame buildings include a plurality of rigid portal frames spaced from each other longitudinally of the building and extending transversely of the building from side wall to side wall. The rigid portal frames include a pair of upstanding side column-s positioned at opposite sides of the building, and a pair of inclined roof members each having one end rigidly joined to the upper end of one of the columns to constitute a knee of the frame, with the other or upper ends of the roof members being joined together at the ridge of the frame. The inclined roof members together comprise a rigid rafter, all portions of the portal frame comprising the rafter member and the two columns being rigid relative to each other in the assembled metallic building structure. Girts and purlins are secured to the columns and to the rafter members, respectively, for interconnecting the rigid portal frames and for supporting the siding and roofing of the building.

Considering only the dead load to which the building is subjected, namely, the weight of the building itself plus any snow load, the principal stresses in the rigid portal frame are bending stresses. As is well known, in buildings of this type, the lower ends of the column can be considered as pivot points for the rigid frames, so that the bending moments in the fname can be taken to be zero at the lower end of the columns and to increase progressively to maximum values at the upper ends of the columns.

These bending moments about the knee of the rigid frame are taken to be negative in sign, that is, the load of the building acts in a direction such as would decrease the obtuse angle between the column and the lower outer portion of the rafter member, or haunch as it is known in the art, if the parts were other than rigidly joined. Thus, the upper and outer sides of the rigid frame in the region of the knee are in tension and the opposite sides are in compression.

Passing from the knee toward the ridge of the rafter member, the negative bending moment decreases from a maximum value adjacent the knee to Zero at a point intermediate the knee and the ridge. From that point, on to the ridge itself, the bending moment changes in sign from negative to positive and attains a maximum positive Value near the ridge. As the rigid frame is bisymmetric about the longitudinal midplane of the building, the pattern of bending moment is exactly the same on the other side of the building and need not be discussed in detail. Just as in the region of negative bending moment adjacent the knee the upper and outer side of the frame was in tension and the inner side was in compression, so also in the region of positive bending moment adjacent the ridge, the upper side of the rafter member is in compression and the lower side is in tension, which is to say that the dead load bending stresses would tend to increase the obtuse angle between the rafter sections that come together at the ridge if they were other than rigidly joined.

The column and rafter sections are ordinarily of I-beam cross section and have webs disposed in vertical planes, with flanges extending lengthwise of the sections along opposite longitudinal edges of the webs. The flanges extend equal distances on opposite sides of the plane of the Webs and are perpendicular to the webs.

The prior art teaches ways of conserving the material of these webs so as to provide structural strength where it is needed but not where it is not needed. To this end, the art teaches the formation of the webs of a depth or width greatest in regions of maximum bending moment and least in regions of least or zero bending moment. Thus, for example, the art teaches that the column Webs can be tapered from a least width adjacent the bottom of the column or point of zero bending moment to a greatest width adjacent the top of the column or greatest region of bending moment at the knee of the rigid frame. The art also teaches, with regard to the rafter member, that the web have a greatest depth adjacent the knee, tapering to a least depth at the point of revensal of the sign of the bending moment and then increasing again to substantial depth at the ridge, so that the depth of the web over the length of the column and the rafter varies directly as the bending moment regardless of the sign of that moment.

Thus to vary the web depth has made possible savings in material by eliminating material where it is not needed for strengthening the structure. However, variation in web depth is by no means the complete answer to the problem of saving material; and the present invention comprises another way to save material, in lieu of but preferably in addition to the technique of varying the web depth.

Accordingly, it is an object of the present invention to provide novel metallic building structure designed to give strength where needed but not where not needed.

Another object of the present invention is the provision of metallic building structure designed to use a minimum of material.

Still another object of the present invention is the provision of rigid portal frames and rafter members for use in such rigid portal frames, designed to achieve the above objects of the invention.

Finally, it is an object of the present invention to provide metallic building structure such as rigid portal frames or rafters for use with rigid portal frames, which will be relatively simple and inexpensive to manufacture,

. easy to install and maintain, and rugged and durable in FIGURE 3 is an enlarged elevational view of half of a rigid portal frame according to the present invention; and

FIGURES 4 and 5 are force diagrams demonstrating certain unobvious benefits of the present invention.

In general, the present invention comprises the discovery that the quantity of material in a rigid portal frame can be substantially reduced if the cross-sectional area of the flanges of the members that make up the rigid frame is specially controlled. Specifically, the crossa sectional area of the lower flanges adjacent the ridge is made substantially less than that of the upper flanges adjacent the ridge; but adjacent the knees, the crosssectional area of the upper or outer flanges is made substantially less than the cross-sectional area of the lower or inner flanges. By this construction, not only is it true that the flanges that are in compression are made heavier than those that are in tension, but more particularly, certain more subtle advantages flow from such control of the cross-sectional areas of the flanges as will appear hereinafter.

Referring now to the drawings in greater detail, there is shown in FEGURE 1 a metallic building the principal dead weight of which is carried by a plurality of rigid portal frames 1 spaced apart lengthwise of the building and resting at their lower ends on concrete flooring 3 to which they are secured for example by anchor bolts (not shown). Supported on the rigid portal frames are a plurality of horizontal siding supporting members in the form of girts 5 and a plurality of roofing supporting members in the form of horizontal purlins 7. The usual bracing rods 8 and other members such as sag rods and the like complete the rigid framework for the building in the generally conventional manner. To this rigid framework are then applied side and end wall sheeting 9 and roof sheeting 11 to complete a weather-tight metallic building.

Each rigid portal frame 1 comprises a pair of columns 13 which rest at their lower ends on flooring 3 and carry between them at their upper ends a rafter member comprising first and second rafter portions. The first rafter portions are each in the form of a haunch 15 secured at its lower outer end to the upper end of column 13. The second rafter portions are in the form of center beams 17 each secured at its lower outer end to the upper inner end of a haunch 15 and at its upper inner end to the other center beam 17 at the ridge of the portal frame.

Each column 13 comprises a flat uniplanar web 19 disposed in a vertical plane extending transversely of the building. Web 19 has an inner flange 21 extending along the inner longitudinal edge thereof and an outer flange 23 extending along the outer longitudinal edge thereof and a horizontal cap plate 25 supported at the top of web 19 and flanges 21 and 23. Flanges 21 and 23 and cap plate 25 are all perpendicular to the plane of web 19 and extend equal distances on opposite sides thereof. Flanges 21 and 23 diverge from each other upward, so that web 19 is tapered as seen in FIGURE 3. The minimum distance between flanges 21 and 23 is at the bottom thereof and the maximum distance between them is at the top thereof.

Haunch 15 has a web 27 disposed in the same vertical plane as web 19, web 27 having an upper flange 29 along the upper longitudinal edge thereof and a lower flange 31 along the lower longitudinal edge thereof. A stiffener 33 in the form of a diagonal web is secured to web 27 and reinforces the knee of the rigid portal frame. An end plate 35 is secured to the lower outer end of web 27. Flanges 29 and 31, stiffener 33 and end plate 35 are all perpendicular to the plane of web 27 and extend equal distances on both sides of that plane. Stiifener 33 may be in two pieces, one welded to each side of web 27. Nut and bolt assemblies 37 rigidly interconnect the flange 23 of column 13 with end plate 35 of haunch 15, and cap plate 25 of column 13 with lower flange 31 of haunch 15, thereby to secure column 13 and haunch 15 in rigid assembly with each other and to provide a rigid knee adjacent the juncture of column 13 and haunch 15. Flanges 29 and 31 converge in a direction away from that knee, so that web 27 is correspondingly tapered.

Center beam 17 has a vertically disposed web 39 lying in the same vertical plane as webs 19 and 27, and carrying on its upper and lower longitudinal edges an upper flange 41 and a lower flange 43. A fishplate connection 45 rigidly interconnects the adjacent ends of haunch 15 and center beam 17 by interconnecting webs 27 and 39, and at the other or upper end of center beam 17, an end plate 47 provides means for connecting the half of a rigid portal frame shown in FIGURE 3 with an identical but bodily reversed half at the ridge of the frame. Flanges 41 and 43 converge toward haunch 15, so that web 39 of center beam 17 is correspondingly tapered.

The essence of the present invention is that the flanges on opposite sides of at least some of the webs are of distinctively different cross-sectional area. Specifically, flange 43, the lower flange of center beam 17, has a crosssectional area substantially less than upper flange 41 thereof. But on the haunch 15, it is upper flange 29 that has a substantially smaller cross-sectional area than lower flange 31. Also, outer flange 23 of column 13 has a substantially smaller cross-sectional area than inner flange 21. Preferably, and in the illustrated embodiment, the differential flange cross-sectional area is achieved by altering flange thickness, so that flange 43 is substantially thinner than flange 41, flange 29 is substantially thinner than flange 31, and flange 23 is substantially thinner than flange 21.

As indicated above, the bending moment along the rafter changes in sign between the knee and the ridge. The rafter is not necessarily designed, however, so that the point of inflection will coincide with connection 45; and in fact, in the rafter shown, the point of inflection actually is intermediate the ends of haunch 15, somewhat closer to connection 45 than to the knee. Thus, the point of minimum web depth lies in a region of positive bending moment, but the actual bending moment at connection 45 is still so small that it does not severely stress connection 45. Thus, in the regions of maximum bending moment at opposite ends of the rafter, the thicker flange is the flange that is in compression and the thinner flange is the flange that is in tension; and just as the compressiontension relationship of the flanges alters intermediate the length of the rafter, so also the differential flange relationship alters intermediate the ends of the rafter, although not necessarily at the same point, as explained above. It is particularly advantageous that the thicker flange be in compression and the thinner flange in tension, since structural steel flanges are much stronger in tension than in compression. It is also to be noted that inner flange 21 of column 13 is not only the flange that is in compression but also the thicker of flanges 21 and 23.

Therefore, it is largely true of the present invention that the thicker flange bears the compressive stresses and the thinner flange the tensile stresses, and that the relationship of the flanges to each other reverses or inverts along the length of the rafter thereby generally preserving this relationship between flange thickness and flange stress. However, a much subtler set of advantages also obtains from this flange reversal or inversion, and these advantages will become apparent from a study of FIGURES 4 and 5 of the drawings.

The stresses in the rigid frame that have heretofore been discussed have been the flange stresses due to bending stress. In addition to these stresses, there are compressive stresses that can be considered to be applied axially of the columns and rafters and which are due to the reaction at the bases of the columns. These compressive stresses are particularly significant in the case of wide, low buildings with low-pitch roofs. In FIG- URES 4 and 5, the principal lines of application of these compressive stresses are indicated by the heavy lines that extend vertically from the base and then diagonally upward at a low pitch in each figure.

The heavy vertical lines in FIGURES 4 and 5 are self-explanatory. The diagonally upwardly extending heavy lines, however, must be viewed as the resultant of its components. The vertical component V is the component applied in the same direction as the column compressive stresses and can be disregarded hereafter. The horizontal component H is the significant factor from a standpoint of determining the bending moment at the knee of the rigid frame, for the bending moment at the knee varies as the product of H and the height of the knee Y. Thus, the compressive stresses are not distinct from the bending stresses, for the horizontal component of the compressive stresses in the rafter is a factor of the bending moment at the knee.

The magnitude of H, however, should not be confused with the horizontal distance between the column and the ridge; in other words, FIGURES 4 and 5 cannot be superposed on an elevational view of a rigid portal frame. This is because although the factor Y varies directly as the height of the column, the factor H also varies to some extent with the height of the column even though the length and pitch of the rafter may remain constant. Specifically, the magnitude of H decreases somewhat with an increase in Y, all other factors being equal. However, the product of H and Y increases with an increase in Y despite the decrease in H, so that it follows that the increase in Y affects the bending moment at the knee at a greater rate than the corresponding decrease in H.

Considering now FIGURE 4 in particular, there is graphically illustrated the effect of making the lower flange of center beam 17 of smaller cross-sectional area than the upper flange thereof. In other words, FIG- URE 4 segregates the effect of unbalancing the flanges to the right of connection 45 as seen in FIGURE 3. This unbalancing of the flanges displaces the neutral axis of center beam 17 upward with no change in column height, that is, no change in Y. This upward displacement of the neutral axis, however, produces a smaller H analogous to the effect of increasing the height of the building. A reduction in H with no increase in Y results in a smaller product HY, that is, a smaller bending moment at the knee, which is obviously a desirable result. Viewed graphically, the upward shift of the neutral axis of the center beam can be represented as in FIGURE 4 by the heavy dashed line which has swung upward counterclockwise at the right of FIGURE 4, with the result that the value of H retreats along the abscissa from H to the lower value of H As is seen in FIG- URE 4, there is no change in Y.

The factor of decreasing the cross-sectional area of the upper flange of haunch I5 is separately examined in FIGURE 5. The downward displacement of the neutral axis of the haunch resulting from this differential flange cross-sectional area decreases the value of Y, because the haunch is connected to the knee. As was seen above, an increase in Y produces a decrease in H, so that a decrease in Y produces an increase in H, but not to the same extent that Y decreases. FIGURE 5 reflects only the decrease in Y, not the corresponding increase in H. This is so that FIGURE 5 will not be needlessly complicated; and it should be understood that the decrease in the value of Y along the ordinate from Y to Y is not proportional to the total decrease in Y but represents the effect of the decrease in Y as partially overcome by the increase in H. Viewed in this light, then, the bending moment at the knee decreases in direct proportion to the decrease in Y from the original value of Y to the modified value of Y In short, reducing the lower flange of the center beam decreases the bending moment at the knee from YH to YH and reducing the upper flange of the haunch decreases the bending moment at the knee from Y H to Y H. The effect of these decreases is additive. At first glance, it might be thought that the reversal of the flange relationships along the length of the rafter would cause the two rafter sections to balance each other so that one would cancel out whatever effect the other one had. In fact, however, the above stress analysis shows Accordingly, the essence of the present invention is the flange area relationship reversal along the length of the rafter, and this reversal produces a unique triple effect, as follows:

(1) It assures that the flanges of greatest cross-sectional area will be in compression and those of least cross-sectional area will be in tension, which desirably reduces the unit area compressive stresses and increases the unit area tensile stresses in the manner the material is best able to bear them;

(2) It decreases the horizontal thrust without a corresponding increase in the moment arm, by shifting the neutral axis of the upper rafter section upward, thereby decreasing the bending moment at the knee; and

(3) It decreases the moment arm at a faster rate than the horizontal thrust increases by shifting the neutral axis of the lower rafter section downward, thereby further decreasing the bending moment at the knee.

In terms of the structural modifications made possible by the present invention, the relatively thin flanges may be viewed as flanges of reduced thickness or reduced cross-sectional area compared to the thicker flanges, which may be left unchanged from their prior art thickness, so that the quantity of material in the frame may be desirably reduced. At the same time, the reduction of the bending moment at the knee enables the quantity of material in the haunch and column to be reduced particularly adjacent the knee. These two reductions in material are additive and coact together to make possible the construction of a rigid frame substantially lighter, less expensive, and easier to manufacture and install than had been possible according to the practices of the prior art.

From a consideration of the foregoing description, it will be obvious that all of the initially recited objects of the present invention have been achieved.

Although the present invention has been described and illustrated in connection with a preferred embodiment, it is to be understood that modifications and variations may be resorted to without departing from the spirit of the invention, as those skilled in this art will readily understand. Such modifications and variations are considered to be within the purview and scope of the present invention as defined by the appended claims.

What is claimed is: 1. A rigid portal frame comprising first and second spaced upright columns and a rafter structure connected to the columns, the rafter structure including a first rafter member extending between one knee of the frame and the ridge of the frame and a second rafter member ex tending between the other knee of the frame and the ridge, the first rafter member and the second rafter member each including a first portion extending from its respective knee and a second portion between the first portion and the ridge, the first portion including an upper flange and a lower flange, the second portion including an upper flange and a lower flange, the upper flange of the first portion and the upper flange of the second portion being substantially coplanar,

and the cross-sectional area of the upper flange of the second portion being substantially greater than the cross-sectional area of the lower flange of the second ridge and in which the second portion is tapered in a direction away from the ridge. I

3. A rigid portal frame as defined in claim 1 in which the cross-sectional area of the lower flange of the first portion is substantially greater than the cross-sectional area of the upper flange of the first portion.

4. A rigid portal frame as defined in claim 3 in which 5 the first portion is tapered in a direction toward the ridge and in which the second portion is tapered in a direction away from the ridge.

References Cited inthefile of this patent UNITED STATES' PATENTS Widrner Jan. 14, 1918 Simpson et a1. Feb. 3, 1959 FOREIGN PATENTS,

Great Britain July 19, 1935 Great Britain Nov. 20, 1947 

1. A RIGID PORTAL FRAME COMPRISING FIRST AND SECOND SPACED UPRIGHT COLUMNS AND A RAFTER STRUCTURE CONNECTED TO THE COLUMNS, THE RAFTER STRUCTURE INCLUDING A FIRST RAFTER MEMBER EXTENDING BETWEEN ONE KNEE OF THE FRAME AND THE RIDGE OF THE FRAME AND A SECOND RAFTER MEMBER EXTENDING BETWEEN THE OTHER KNEE OF THE FRAME AND THE RIDGE, THE FIRST RAFTER MEMBER AND THE SECOND RAFTER MEMBER EACH INCLUDING A FIRST PORTION EXTENDING FROM ITS RESPECTIVE KNEE AND A SECOND PORTION BETWEEN THE FIRST PORTION AND THE RIDGE, THE FIRST PORTION INCLUDING AN UPPER FLANGE AND A LOWER FLANGE, THE SECOND PORTION INCLUDING AN UPPER FLANGE AND A LOWER FLANGE, THE UPPER FLANGE OF THE FIRST PORTION AND THE UPPER FLANGE OF THE SECOND PORTION BEING SUBSTANTIALLY COPLANAR, AND THE CROSS-SECTIONAL AREA OF THE UPPER FLANGE OF THE SECOND PORTION BEING SUBSTANTIALLY GREATER THAN THE CROSS-SECTIONAL AREA OF THE LOWER FLANGE OF THE SECOND PORTION AND OF THE CROSS-SECTIONAL AREA OF THE UPPER FLANGE OF THE FIRST PORTION. 